CI: fit the dependency set on the runner's disk

uv's cache moves into the workspace so it shares a filesystem with
.venv and hardlinks instead of double-storing ~1.3 GB per job (the
'Failed to hardlink, falling back to full copy' warning was the tell
before ENOSPC), and both matrices run max-parallel 1 so the host
holds one dependency set at a time instead of six.

.gitea/workflows/ci.yml

@@ -22,6 +22,10 @@ on:
 
 env:
   UV_PYTHON: "3.12"
+  # Cache on the same filesystem as .venv so uv hardlinks instead of
+  # double-storing ~1.3 GB per job (PySide6 + scipy + matplotlib) —
+  # the runner host ran out of disk with the default cache location.
+  UV_CACHE_DIR: .uv-cache
   # No test opens a window, but if one ever creates a figure the Qt
   # backend must not try to reach a display on a headless runner.
   QT_QPA_PLATFORM: offscreen
@@ -32,6 +36,9 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
+      # Serialized: the runner host has limited disk; one dependency
+      # set at a time keeps peak usage at a single job's worth.
+      max-parallel: 1
       matrix:
         python: ["3.11", "3.13"]
     env:
@@ -55,6 +62,8 @@ jobs:
     container: ${{ matrix.distro }}
     strategy:
       fail-fast: false
+      # See the linux job: serialized for the runner host's disk.
+      max-parallel: 1
       matrix:
         include:
           # git in every list: the checkout below is plain git because